Known Imaging Radar Principles
FIG. 1 shows an example of conventional imaging radar apparatus orbiting the Earth. As shown in the Figure, the imaging radar is an active instrument that illuminates the ground through a narrow beam antenna which is also used to receive the radiation scattered back by the ground. Ground illumination is by a sequence of short radar pulses, repeated typically at a rate of a few kHz, as the antenna moves steadily along track. The received radar pulse reflection sequence is sampled at a high rate and can then be processed to form maps of the ground radar reflectivity (images) and other ground information products.
The radar Operator is obliged to provide image data for the ground regions defined by the Customer(s). The radar functions by accumulating reflection data as its beam footprint moves over the desired ground region. In order to ensure that the correct piece of ground is illuminated by the radar, the antenna pointing direction must be known and controlled. The region illuminated by the radar is larger than the ordered image dimensions by a margin of approximately 10% (using a greater margin than this would be energy inefficient). Thus, the beam pointing accuracy is typically of the order of 1/10th the radar beam-width in both the along track and across track senses.
Radar Operation for Image Formation
As is well known in the art of imaging radar, the radar pulse characteristics required for attitude determination differ from the characteristics necessary for image formation. A brief description of known SAR imaging pulse characteristics is given here as background.
The slant range resolution required of a SAR instrument for adequate image formation is typically significantly less than 100 m (and in some cases significantly less than 1 m). The duration of a pulse of un-modulation microwave carrier capable of this slant range resolution is typically sub-microsecond (the required spatial resolution sets the necessary pulse bandwidth). In order to achieve adequate radiometric resolution, each ground resolution cell must also be illuminated with sufficient energy to ensure that the radiation it scatters back to the antenna is received with sufficient signal to noise ratio (SNR).
The above requirements taken together demand an un-modulated carrier pulse transmission power far higher than is practicable for a non-ground based radar system. However, a ground resolution cell can be illuminated with the same energy by using a lower power pulse but which is of correspondingly longer duration without changing the pulse bandwidth (resolution). This can be achieved in practice by modulating the microwave carrier by a tone whose frequency is ramped (usually linearly). The extent of the frequency ramp applied to the carrier is the pulse bandwidth. In this way a relatively long pulse radiation pulse is given large bandwidth (resolving power).
Thus, each SAR pulse is elongated and of moderate power, consisting of a carrier modulated by a tone of ramping frequency (sometimes called a “chirp”), and is capable of resolving ground features separated (across track) by distances determined by its slant range resolution and local angle of incidence.
Along track resolution is achieved by synthesising a large along track aperture from a sequence of radar pulses. Such synthesis is a coherent signal processing operation which extracts along track image information from the phase progression of radar reflections between successive pulses, using knowledge of the viewing geometry. For simplest processing the radar must generate a pulse every time the antenna along track position advances by the same particular distance. For many known SAR systems (space-borne in particular) this corresponds to pulse emission at equally spaced time intervals (usually less than 1 ms). Each pulse typically has the chirp characteristic as described hereinabove. In the case of a known space SAR (where the round trip delay between pulse transmission and reception of the reflection is typically a few milliseconds), several pulses are in flight simultaneously.
Received pulse reflections are typically demodulated by the radar and sampled at a rate consistent with the required slant range resolution (from 10's to 100's of MHz). Except in the case of the most demanding requirements for real-time in-flight imagery, the image is computed from the acquired data stream on the ground some time after it is acquired. Conventional SAR image processing is thus extremely computationally intensive.
Furthermore, conventional imaging radar typically relies upon the operation of a whole suite of attitude control system sensors (for example, star sensors from a space-based mission) to provide the necessary level of attitude knowledge for enabling adequately accurate pointing of the radar beam. This leads to cost disadvantages and to a high processing load.